1. Field of the Invention
This disclosure relates to an image forming apparatus capable of computing a stacking amount of sheets stored in a sheet storage portion.
2. Description of the Related Art
An image forming apparatus of the related art such as printers, copying machines, and facsimiles is provided with a sheet feeding apparatus configured to send forth a sheet stored in a sheet storage portion demountably mounted on an image forming apparatus body by a sheet feed portion such as a pickup roller and feed the sheet to an image forming portion. Examples of the sheet feeding apparatus include a sheet stacking portion provided in the sheet storage portion so as to be movable upward and downward, and configured to, when feeding the sheet, lift the sheet stacking portion to press the sheet against the sheet feed portion and, in this state, rotate the sheet feed portion to feed the sheet.
In the sheet feeding apparatus, the height of a topmost sheet stacked in the sheet stacking portion needs to be maintained at a predetermined height allowing the sheet feed portion to feed the sheet. Therefore, an elevating portion configured to move the sheet stacking portion upward and downward is provided in the sheet storage portion, and the image forming apparatus body is provided with a drive portion and a drive transmitting mechanism configured to transmit drive of the drive portion to drive the elevating portion. When the height of the topmost sheet is lowered, the drive portion is driven, the drive of the drive portion is transmitted to the elevating portion by the drive transmitting mechanism to lift the sheet stacking portion, and the height of the topmost sheet is maintained at a predetermined height allowing the sheet feeding.
The sheet feeding apparatus of this configuration includes a sheet surface position detection sensor configured to detect the height of the topmost sheet. When the sheet storage portion is set to the image forming apparatus body, the sheet feeding apparatus inspects a state of the sheet surface position detection sensor first, and if the sheet surface position detection sensor is OFF (non-detecting state), lifts the sheet stacking portion until the sheet surface position detection sensor is turned ON.
Japanese Patent Laid-Open No. 2000-289861 discloses a configuration adapted to count time from the start of lifting of the sheet stacking portion until the sheet surface position detection sensor is turned ON, and compute a sheet stacking amount in the sheet stacking portion, that is, the remaining amount on the basis of a counted value and a lifting speed of the sheet stacking portion. When computing the sheet stacking amount from a lifting time corresponding to time until the sheet surface position detection sensor is turned ON and a lifting speed in this manner, a control unit computes the stacking amount in accordance with an arithmetic expression for computing the sheet stacking amount set in advance. The arithmetic expression is generally expressed by a linear function as H=−at+b, where t is lifting time, and H is a stacking amount.
However, in the case where the relationship between the lifting time and the lifting speed varies in the middle of the upward movement, if the stacking amount is computed by the linear function, there arises a difference between the computed sheet stacking amount and an actual stacking amount, and accurate detection of the stacking amount is not achieved.